Conventionally, a concentrated winding motor used for a compressor of an air conditioner, a refrigerator, and so on, is configured as shown in FIGS. 6 to 8.
The concentrated winding motor is constituted by a stator 60 and a rotor 80, which is rotatably supported inside the stator 60. The stator 60 of FIGS. 6 and 8 is formed by stacking stator-cores 61. The stator-core 61 is formed by teeth 62, each having a winding thereon, and a yoke 64, which is substantially annular and connects the outer peripheral surfaces of the teeth 62. On the ends of the teeth 62, teeth projecting tips 65 are formed so as to protrude in a peripheral direction along the internal diameter of the stator-core.
Windings provided the six teeth 62 formed on the stator-cores 61 are indicated by three-phase windings 63U, 63V, and 63W in FIG. 6 but the shapes of the three-phase windings 63U, 63V, 63W are not specifically shown in FIG. 6. To be specific, the windings are wound as shown in FIG. 7.
FIG. 7 is a sectional view taken along a line X-X′ of FIG. 6. Windings 63 representing the three-phase windings 63U, 63V, and 63W are wound around the teeth 62 of the stator-cores 61 via insulating materials 67, each being composed of an insulator formed as a film or a resin.
The three-phase windings 63U, 63V, and 63W make star connection each other and produce 120°-rectangular wave driving, in which two of the three phases are simultaneously driven while being brought into excitation. Further, applied voltage is changed by PWM control.
Moreover, the stator-cores 61 having such shapes are stacked in a straight line along the axial direction without forming skew. Notches 66 formed on the outer periphery of the stator-cores 61 act as through holes between a shell 90 and the stator-cores 61 and act as passages of refrigerant in a state in which the stator 60 is shrink-fitted into the shell 90 of a compressor.
The rotor 80 is rotatably held in the stator 60 concentrically with the stator 60. The rotor 80 has permanent magnets 82 embedded into a rotor core 81. End plates (not shown) are placed on both ends of the rotor core 81, and a rivet (not shown) is passed through a through hole made in the rotor core 81 to be caulked, so that the end plates on the ends are fixed. Further, a shaft passes through a shaft hole 83.
Therefore, because of a rotating magnetic field generated by current applied to the three-phase windings 63U, 63V, and 63W that are provided on the stator 60, the rotor 80 is rotated by torque, which is generated by combining magnet torque and reluctance torque, centering around the shaft.
As described above, on the stator 60 formed by stacking in a straight line along the axial direction without forming skew, since attracting stress or repulsing stress increases between the adjacent teeth projecting tips 65, vibration is more likely to increase as compared with distributed windings. This is largely affected by vibration in a radius direction as well as vibration in a rotational direction.
Particularly in the case of PWM (Pulse Width Modulation) control and 120°-rectangular wave driving, in which only two of three phases are brought into excitation, vibration remarkably increases. This is because current applied to windings includes more harmonics as compared with sinusoidal driving. Besides, in 120°-rectangular wave driving, since current abruptly changes, strong exciting force is generated on the teeth projecting tips, thereby increasing vibration.
It has been conventionally known that the formation of skew is effective as a method for reducing irregularities of torque and reducing vibration. The formation of skew is not limited to a stator. For example, Japanese Patent Laid-Open No. 2000-175380 discloses that skew is formed on a rotor or both of a stator and a rotor. It is possible to reduce vibration generated on the concentrated winding motor shown in FIG. 6 by forming skew.
In the case where irregular torque is reduced by forming skew on a stator and a stator-core has ribs for forming teeth on the internal diameter of an annular ring for forming a yoke thereon, the ribs are inclined by the skew so as to reduce the occurrence of annular vibration. However, simply by forming skew on the stator, vibration cannot be completely eliminated. Further, it has been found that in the slightly generated annular vibration, windings stored in the same groove for windings in a non-contact state become mass, resulting in larger vibration.